Assembly Considerations
Molded-in metal inserts can cause high residual stresses in plastic bosses. Avoid inserts in parts made of polycarbonate resins and blends, because the residual stress may result in crazing, cracking, and eventual part failure. Plastic, having much higher coefficients of thermal expansion than metal, shrinks around the insert and becomes stressed at the interface because the insert imposes a restriction. Because glass-reinforced resins have thermal expansion coefficients closer to those of metals, problems with metal inserts occur less frequently in these resins. Molded-in metal inserts have also been used successfully in various grades of thermoplastic urethanes. Always thoroughly test all molded-in inserts in end-use conditions prior to beginning full production runs. Before inserts are placed in a mold, they should be cleaned to remove foreign matter, including any oils or lubricants. Inserts should seat securely in the mold to prevent floating and possible damage to the mold. Avoid inserts with sharp knurls or protrusions. Although they can have high pullout values, the sharp points cause a notch effect in plastics that can lead to early failure. Inserts larger than 0.25-inch (6.35-mm) diameter may induce excessive thermal stresses, which can be partially reduced by preheating the insert prior to placing it in the mold. Preheat inserts used in polycarbonate parts to 350°F to 400°F (177°C to 204°C). |